1) Put self-raising flour, baking powder, sugar and cocoa in a bowl and mix well.
2) Pour over the vegetable oil and vanilla essence and then gradually add the water.
3) Mix thoroughly with a wooden spoon or electric whisk. Add the chocolate chips at this stage if you are using them.
4) Pour into shallow oblong tin (7"x11") and cook for about 25 minutes at 200ºC/400ºF until cooked.
5) For the icing, melt margarine and add cocoa. Simmer for a couple of minutes over very low heat.
6) Take off the heat. Add a couple of tablespoons of soya milk, and begin to add icing sugar through a sieve (or it gets pretty lumpy).
7) Continue to add icing sugar and stir, adding more soya milk as necessary.
8) Stir in vanilla and spread onto cake while it is still warm.